The present invention is directed to talc-filled thermoplastic polyester compositions which have improved stability. In particular, the present invention is directed to stabilizing thermoplastic polyester compositions which contain a talc filler and which are further modified by the incorporation of a butadiene polymer to provide improved impact strength to the polyester.
Thermoplastic polyesters such as those formed from an aliphatic or cycloaliphatic diol, or mixtures thereof, containing from 2 to about 10 carbon atoms and an aromatic dicarboxylic acid as well as segmented thermoplastic copolyester elastomers have long been known in the art and have been used in the preparation of molded and extruded articles. Such resins can be used alone or in conjunction with fillers or other resins to provide materials having a variety of properties. Thus, it is known to incorporate impact modifiers such as butadiene-based core-shell type impact modifiers into poly(C.sub.2 -C.sub.4) alkylene terephthalates. It is also known to add talc to thermoplastic polyester compositions in order to improve the thermal and shrinkage properties of the composition. Examples of adding a talc filler to thermoplastic polyesters are disclosed in U.S. Pat. Nos. 3,859,246; 4,421,888 and 4,731,407. It is further well known that thermoplastic polyesters including the segmented copolyester elastomers are subject to thermal degradation and that the unstabilized polymers exhibit poor processing behavior and unacceptable performance, especially at elevated temperatures. A number of antioxidant stabilizers are available for use in these polymers. For example, U.S. Pat. Nos. 4,520,148; 4,520,149 and 4,520,150 disclose stabilizing thermoplastic segmented copolyester elastomers with various stabilizer combinations. It is also well known to include talc into thermoplastic polyester compositions and to stabilize the composition with known antioxidants.
However, it has recently been discovered by the present inventors that the incorporation of talc into thermoplastic polyesters resins, in particular, thermoplastic polyesters resins which have been modified by the incorporation of a butadiene-based impact modifier, causes the degradation of the polyester during processing of the composition. Thus, it has been found that these talc-filled thermoplastic polyester compositions degrade during the mixing of the composition as well as during further processing operations such as during extrusion and injection molding. Unfortunately, the addition of common antioxidants or other stabilizers known to be effective stabilizers for polyester resins do not prevent the degradation of the talc-filled polyester resins which have been modified with the butadiene-based impact modifiers.
Accordingly, it is a primary objective of the present invention to improve the stabilization of talc-filled thermoplastic polyesters. Another object is to improve the stabilization of such talc-filled polyesters which have been further modified with a butadiene-based impact modifier.